Thin, high leadcount package

ABSTRACT

A high leadcount surface mount IC package. The IC package of the present invention includes a plastic molded compound which is localized over the surface of the die, such that it covers the interconnections of the die. The package includes a leadframe having many leads. The leads are supported using a ring support structure.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuits; moreparticularly, the present invention relates to a package for a thin,high lead count surface mount integrated circuit.

BACKGROUND OF THE INVENTION

Packaging of electronic devices provides mechanical support, protectionof the electronic circuitry components, and a medium for interconnectingthe chip to a circuit board for use in a system. As systems becomesmaller, a need exists to make packages as small as possible.

The packaging process begins with the fabrication of a crystallinesemiconductor material, usually silicon or gallium arsenide. Individualdies (i.e., chips) are formed on the wafer at the same time. Then thewafer is separated into single dies. One limit in making small packagesis the size of the dies (e.g., the size of the silicon).

Each of the individual dies is typically packaged in a chip carrier.External connections on the chip carrier package allow for the chip tobe mounted on a printed wiring board. The chip carrier is electricallyconnected to the printed wiring board by surface mounting the chipcarder directly to the mounting surface.

There are a variety of sizes for packages. For instance, thickness ofthe individual packages varies between different packages. For example,while some packages are only 1 mm think, other packages range up to 5 mmthick. Generally, for packages having leads, the smaller the number ofleads that a package has, the thinner the package is itself. Thedisparity in package thicknesses is due to the use of more moldingcompound for packages having a greater number of leads, The addedmolding is required to hold the many leads rigid. Thinner packages donot have some of the reliability problems of larger packages.

The primary problem with larger packages is their susceptibility tocracking during the solder reflow process. The cracking results frommoisture which has been absorbed by the plastic. During the solderreflow process, the package heats up to 240°-260° F., causing theabsorbed water to turn to steam. The steam causes pressure to build upwithin the plastic, and the plastic cracks in order to relieve thepressure (i.e., the steam).

To prevent the cracking of the package, the package is protected frommoisture. One method of protecting the package from the effects ofmoisture is to bake the packages at lower temperatures. By baking thepackage at a lower temperature, the moisture has an opportunity to bereleased by the plastic molding. This baking step is referred to as thepreheat stage. Since the moisture is not trapped in the package,cracking is less likely. Also to prevent the further infiltration ofmoisture into the package, the chips are stored in moisture proof bags.The bags contain desiccant to keep the humidity within the bag to aminimum while the chips are being stored. The desiccant absorbs excessmoisture. However, if the bag has been punctured or has been stored fortoo long a period of time, then the bag is likely to have absorbedmoisture. Whether moisture has entered the storage bag is indicated by acolor change in moisture sensitive material which is also included inthe bag. Even with these precautions, the packages are baked gently uponremoval from the bag before they are mounted on the board to insureagainst moisture and its effects. The performance of these proceduresdoes not ensure that the packages are moisture free. Thus, even thoughthese precautions are undertaken, damaged chips still may be produced.

When packages are thin, they are less likely to crack. Thin packages areless likely to crack because the packages are too thin to hold orcontain a substantial amount of moisture and the preheat stage is enoughto drive all of the moisture out of the package. Therefore, at the hightemperature of the reflow stage, cracking does not occur because thereis an insignificant amount of moisture remaining in the plastic molding.

Another problem associated with larger packages is that larger residualstresses are generated. In other words, large amounts of moldingcompound induce stresses in the die which the structure cannot support.The residual stresses occur in any molding operation and are accentuatedby a mismatch of the thermal expansion between the leadframe, which isgenerally copper, the molding compound, and the silicon, all of whichproduce stresses and warpage. As a result, the package warps, resemblinga "potato chip". Thus, it is desirable to make the package thin andsmall (in the x,y dimension) to decrease the die stresses.

As will be shown, the present invention comprises a thin smallintegrated circuit package. The package of the present inventioncomprises a thin molding compound locally around the die with a separatesupport for the leadframe to keep the leads in position.

SUMMARY OF THE INVENTION

A high leadcount surface mount integrated circuit (IC) package isdescribed. The IC package of the present invention includes a plasticencapsulating molded compound. The molded compound is localized over thedie and its associated electrical interconnects. The IC package of thepresent invention also includes a leadframe having multiple leads. Theleadframe of the present invention is substantially planar. Each of theleads extends into the molded compound and is electrically coupled tothe die by wirebond, tab or other technology. The IC package of thepresent invention also includes a support structure which supports theleads. In the currently preferred embodiment, the support structuresupports the leads in the same plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of thepreferred embodiment of the invention, which, however, should not betaken to limit the invention to the specific embodiments, but are forexplanation and understanding only.

FIG. 1 illustrates the top view of the package of the present invention.

FIG. 1A illustrates a section view of the package containing anintegrated circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A thin, high leadcount integrated (IC) circuit package is described. Inthe following description, numerous specific details are set forth, suchas specific numbers of leads, materials, shapes, etc., in order toprovide a thorough understanding of the preferred embodiment of thepresent invention. It will be obvious to those skilled in the art thatthe present invention may be practiced without these specific details,In other instances, well-known techniques have not been described indetail in order not to unnecessarily obscure the present invention.

FIG. 1 and 1A illustrate the IC package of the present invention.Referring to FIG. 1, die 104 is shown encapsulated by molding compound101. Die 104 is attached to leadframe 103. In the currently preferredembodiment, die 104 is physically mounted to leadframe 103. Leadframe103 comprises a plurality of leads. The leads of leadframe 103 aresupported by support structure 102. FIG. 1A shows the side view of theIC package of FIG. 1 along section arrow lines 1A.

Molding compound 101 is localized around the die in an area onlyslightly larger than die 104. Molding compound 101 only extends to anarea which covers and protects the bond wire or tab interconnect. Insome embodiments, molding compound 101 extends not more than 10% pastthe side of die 104, while in other embodiments, molding compound 101extends 2 mm past the end of die 104. In the currently preferredembodiment, molding compound comprises of plastic.

By having molding compound 101 localized and thinly deposited over thetop of die 104, the stresses on die 104 are reduced. Essentially,because molding compound 101 itself is reduced in size, the stresses ondie 104 are balanced. The silicon is much more rigid than the plastic,such that when the plastic molding compound 101 becomes very thin, thesilicon is able to contribute a larger influence or force on theplastic. Since plastic molding compound 101 is thin, it does not exerttoo much force on the silicon. As a result, the silicon keeps everythingflat, so that die 104 stresses are not large enough to cause warpage.

The thinness of the package of the present invention also permitsmoisture to exit the plastic molding compound more quickly andthoroughly than with thicker packages. A very thin layer of moldingcompound 101 over die 104 helps the package to be resistant to crackingduring solder reflow. Molding compound 101 is able to dry out veryquickly during the pre-heat stage of the process. Therefore, there isless tendency for absorbed moisture, as is the case with thickerpackages, to crack the package due to steam buildup.

Leadframe 103 is constructed from a number of various metals which arewell-known in the art for constructing leads. Leadframe 103 for thepresent invention is flat and is formed by stamping or etching it from adesired metal sheet. With molding compound 101 limited to slightlygreater than the size of die 104, the remainder of the package must besupported. The leads of the package go out to support structure 102.Support structure 102 becomes a permanent part of leadframe 103, servingto support the leads through the package assembly, test and boardassembly. Specifically, the leads must be supported to prevent movementduring the remainder of the manufacturing process. The leads must bealso supported to ensure that all the leads are in the same plane whenbeing mounted to the circuit board. If they are not in the same plane,then some of the leads may not make contact when they are being soldereddown.

In the currently preferred embodiment, the leads of the package aresupported by a ring support structure. In the currently preferredembodiment, the ring structure is molded or bonded prior to theplacement of the die in the package and the molding of the compound(e.g., plastic) around the die. The ring is employed to hold the leadscaptive during package assembly, test and board assembly. In thecurrently preferred embodiment, the ring remains on the package, evenafter attachment of the package to the circuit board. It should be notedthat ring support structure 102 does not add thickness to the rest ofthe package. In the present invention, the thickness of ring supportstructure 102 should be no thicker than the thickness of moldingcompound 101 over the die.

In other embodiments, support structure 102 is laminated to leadframe103 prior to molding. By having support structure 102 attached prior tomolding, it serves as a dam bar to prevent molding compound 101 fromextruding out between the leads when molding compound 101 is injected.

It should be noted that, in the case of the dam bars of the prior art,each of the leads must be punched-out or singulated to preventelectrical shorting. The punching out of each lead is a fine tunedprocess, which is capable of leaving little bits of flash or junk (i.e.,molding compound where it should not be or where it isn't wanted). Inthe present invention, each of the leads does not have to be singulatedif ring or support structure 102 does not produce a short circuitbetween leads.

In the present invention, ring support structure 102 comprises amaterial that provides the desired stiffness for the weight and size ofleadframe 103. In the currently preferred embodiment, ring supportstructure 102 could be constructed out of metal that is insulated fromthe leads. In other embodiments, ring support structure 102 comprises arigid fiberglass material.

Ring support structure 102 could be constructed to extend to the edge ofmolding compound 101. However, if ring support structure 102 and moldingcompound 101 remain separate, then the stresses corresponding to eachare not transferred between the two because the two are only coupled bythe copper traces of leadframe 103. In other embodiments, ring supportstructure 102 extends either all around or underneath the die, such thatmolding compound 101 and die 104 are resting on top of ring supportstructure 102.

Whereas many alterations upon applications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is understood that theparticular embodiment shown and described by illustration is in no wayintended to be limiting. Therefore, reference to details of thepreferred embodiment is not intended to limit the scope of the claimswhich themselves recite only those features regarded as essential to theinvention.

Thus, a very thin, high leadcount integrated circuit package has beendescribed.

I claim:
 1. A high leadcount surface mount integrated circuit package comprising:an integrated circuit die; a substantially planar leadframe having a plurality of leads, wherein each of the plurality of leads includes a first end, a second end and an intermediate portion between the first end and the second end, wherein the first end of each of said plurality of leads is coupled the die in an interconnect area support means for permanently supporting the plurality of leads, wherein the support means is attached to the intermediate portion of each of the plurality of leads, such that the plurality of leads are held in a single plane and each of the plurality of leads is held permanently in a predetermined position with respect to the other of said plurality of leads; and a plastic encapsulating molded compound, wherein the compound encapsulates only the die and the interconnect area and is localized over the die and the interconnect area, wherein the compound only extends to an area that covers the interconnect area and the die, such that said compound is spaced apart from the support means.
 2. The package as defined in claim 1 wherein the support means comprises a ring.
 3. The package as defined in claim 1 wherein said support means comprises a fiberglass ring.
 4. The package as defined in claim 1 wherein said support means comprises a conductive portion and an insulated portion, wherein each of the leads is electrically isolated from the conductive portion by the insulated portion.
 5. The package defined in claim 1 wherein the plastic compound inclusive of the die comprises a first thickness, and wherein the support means has a second thickness, such that the second thickness is less than the first thickness.
 6. The package defined in claim 1 wherein the die is wire bonded to the leadframe.
 7. The package defined in claim 1 wherein the leadframe is coupled to the die by tape automated bonding.
 8. The package defined in claim 1 wherein the die has a top, a bottom and four sides, and wherein the plastic compound is molded around the die such that the plastic compound extends a distance of 2 mm or less beyond each of the four sides of the die along the plurality of leads.
 9. The package defined in claim 1 wherein the die has a top and four sides, wherein the top has a length and a width, and further wherein the plastic compound is molded around the die such that the plastic compound extends a distance beyond each of the four sides of the die along the plurality of leads, wherein the distance is less than or equal to ten percent of the length.
 10. The package defined in claim 1 wherein the support structure is molded to the leadframe.
 11. The package defined in claim 1 wherein the support structure is bonded to the leadframe.
 12. The package defined in claim 1 wherein the support structure is laminated to the leadframe.
 13. A high leadcount surface mount integrated circuit package comprising:an integrated circuit die; a substantially planar leadframe having a plurality of leads, wherein each of the plurality of leads includes a first end, a second end and an intermediate portion between the first end and the second end, wherein the first end of each of said plurality of leads is wire bonded the die in an interconnect area; a support structure for permanently supporting the plurality of leads, wherein the support structure is coupled to the intermediate portion of each of the plurality of leads, such that the plurality of leads are held in a single plane and the intermediate portion of each of the plurality of leads is held permanently in a predetermined position with respect to the other of said plurality of leads; and a plastic molded compound, wherein the plastic compound is deposited over the die and is only localized over the die and the interconnect area, such that the plastic compound only extends to an area that covers the interconnect area and the die, and wherein the plastic compound is spaced apart from the support structure, such that a gap along the plurality of leads between the support structure and the plastic compound is created.
 14. The package defined in claim 13 wherein the die has a top, a bottom and four sides, and wherein the plastic compound is molded around the die such that the plastic compound extends a distance of 2 mm or less beyond each of the four sides of the die along the plurality of leads.
 15. The package defined in claim 13 wherein the die has a top and four sides, wherein the top has a length and a width, and further wherein the plastic compound is molded around the die such that the plastic compound extends a distance beyond each of the four sides of the die along the plurality of leads, wherein the distance is less than or equal to ten percent of the length.
 16. The package defined in claim 13 wherein the plastic compound inclusive of the die comprises a first thickness, and wherein the support structure has a second thickness, such that the second thickness is less than the first thickness.
 17. The package defined in claim 13 wherein the support structure comprises a ring.
 18. The package defined in claim 13 wherein said support structure comprises a fiberglass ring.
 19. A high leadcount surface mount integrated circuit package comprising:an integrated circuit die; a substantially planar leadframe having a plurality of leads, wherein each of the plurality of leads includes a first end, a second end and an intermediate portion between the first end and the second end, wherein the first end of each of said plurality of leads is coupled the die in an interconnect area; a support structure for permanently supporting the plurality of leads, wherein the support structure is coupled to one part of the intermediate portion of each of the plurality of leads, such that the plurality of leads are held in a single plane and the intermediate portion of each of the plurality of leads is held permanently in a predetermined position with respect to the other of said plurality of leads, and wherein the support structure extends underneath the die; and a plastic molded compound, wherein the plastic compound is deposited over the die and is only localized over the die and the interconnect area, such that the plastic compound only extends to an area that covers the interconnect area and the die, such that a portion of each of the plurality of leads in a space between the plastic compound and the part of the intermediate portion is exposed.
 20. The package defined in claim 19 wherein the die has a top and four sides, wherein the top has a length and a width, and further wherein the plastic compound is molded around the die such that the plastic compound extends a distance beyond each of the four sides of the die along the plurality of leads, wherein the distance is less than or equal to ten percent of the length. 